Glutamate is a major neurotransmitter in the brain, and its role in drug addiction remains a hot topic in animal research aimed to unravel neurochemical mechanisms for addictive properties of drugs of abuse. This lab has long been supported for studying roles of metabotropic glutamate receptors (mGluRs). In the last period of grant, we have identified that group I mGluRs (mGluR1 and 5 subtypes) that are densely expressed in the striatum are key regulators of drug action. Specifically, activation of group I mGluRs is required for acutely- administered psychostimulant amphetamine (AMPH) to enhance endogenous dynorphin activity in rat striatal neurons, a critical response that was thought to homeostatically inhibit drug effects. One form of long-lasting behavioral plasticity, behavioral sensitization to repeated AMPH injections in rats, is a widely-used animal model because it mimics the intensification of drug craving in human addicts. Interestingly, we have recently found that repeated AMPH administration markedly reduced group I mGluR mRNAs and proteins in the striatum. This raises an innovative question as to whether repeated AMPH injections could progressively downregulate the group I mGluR expression and thus function in activating inhibitory dynorphin and thereby induce behavioral sensitization. In this continuation proposal, a series of experiments was therefore proposed to evaluate an overarching hypothesis that the downregulation of group I mGluRs contributes to behavioral sensitization to repeated AMPH administration. Using multidisciplinary approaches, this hypothesis will be tested in rodents in vivo in three AIMs. In these AIMs, we will (1) explore and characterize the reduction of mGluR1 and mGluR5 protein expression in the striatum in response to repeated AMPH administration, and elucidate cellular mechanisms for such reduction, (2) define functional consequences of reduced mGluR1/5 expression in terms of mGluR1/5-mediated functions by examining effects of repeated AMPH administration on the prime mGluR1/5-mediated cellular and genomic responses in striatal neurons, including phosphoinositide hydrolysis, intracellular Ca2+ release, MAPK/ERK activation, transcription factor (CREB) phosphorylation, and dynorphin gene expression, and (3) define functional roles of the downregulated mGluR1/5 in behavioral sensitization by examining behavioral sensitization to repeated AMPH administration in rats in which reduced mGluR1/5 protein expression is reversed or restored by viral-mediated transgene expression or by proteasome inhibitors, or in rats in which mGluR1/5 proteins are experimentally reduced by an antisense or genetic approach. Results achieved here from molecule to behavior will provide evidence for a new molecular mechanism underlying drug action, and will ultimately contribute to the development of novel pharmacotherapies, by targeting the group I mGluRs, for the treatment of various mental illnesses stemmed from substance abuse. PUBLIC HEALTH RELEVANCE: Substance abuse and addiction is among major social, economic, and health problems. This application is proposed to discover brain mechanisms critical for the addictive properties of drugs of abuse. The findings through this research project will promote the development of novel pharmacotherapies for the treatment of various mental illnesses stemmed from substance abuse.